1. Field of the Invention
The present invention relates to an optical waveform shaper shaping the waveform of optical signals without conversion into electrical signals.
2. Description of the Related Art
In recent years, structures or methods utilizing optical signals have been applied to fields such as communications, signal processing and measurement. Such optical technology seems to show promise of further advances in the future.
In order to improve the S/N ratio of optical signals, it is necessary to reshape the waveform of the optical signal. In optical communication systems, for example, the optical signal transmitted by a transmitter is attenuated with the transmission. Therefore, the waveform of a signal arriving at a receiver is distorted compared with that of the signal transmitted by the transmitter. As a result, the S/N ratio is degraded, posing the potential problem that the receiver cannot demodulate/decode the received signal correctly.
Technology to improve the S/N ratio of optical signals, structures utilizing a Nonlinear Optical Loop Mirror (NOLM) (see Non-patent Document 1, for example) and structures utilizing a wavelength converter (see Non-patent Document 2, for example) are widely known.
The NOLM is, as shown in FIG. 1A, comprised of a directional coupler, a highly nonlinear optical fiber and an optical attenuator. The optical input is split by the directional coupler. In FIG. 1A, one output of the coupler is propagated clockwise in the highly nonlinear optical fiber, and the other output is propagated counterclockwise in the highly nonlinear optical fiber after attenuated by the optical attenuator. These two outputs are coupled by the directional coupler, and the result is output. The output is the regenerated optical input with its waveform shaped. Additionally, Non-patent Document 1 describes an experimental result demonstrating that the waveform-shaping effect can be improved by connecting the NOLM serially.
The wavelength converter, as shown in FIG. 1B, comprises a plurality of semiconductor optical amplifiers (SOA). The optical signal and optical continuous wave (CW) with a wavelength different from the wavelength of the optical signal are input to one semiconductor optical amplifier, and the optical CW alone is input to the other semiconductor optical amplifier. By combining the outputs of these semiconductor optical amplifiers, the wavelength of the optical signal is converted based on the optical CW input. The converted output signal has its waveform shaped. Non-patent Document 2 describes a configuration in which semiconductor optical amplifiers are connected serially.
The following Patent Documents 1 through 4 describe known technologies relating to waveform shaping or pulse regeneration.    <Patent Document 1> Japanese unexamined patent publication bulletin No. 07-321742 (FIG. 10)    <Patent Document 2> Japanese unexamined patent publication bulletin No. 09-133825 (FIG. 1 through FIG. 3)    <Patent Document 3> Japanese unexamined patent publication bulletin No. 10-293332 (FIG. 17 through FIG. 21)    <Patent Document 4> Japanese unexamined patent publication bulletin No. 2003-186067 (FIG. 14)    <Non-patent Document 1> B. K. Nayar, et al. 1993. Concatenated All-optical Loop Mirror Switches. Journal of Modern Optics 40(12): 2327-2332.    <Non-patent Document 2> K. E. Stubkjaer et al. Wavelength Conversion Device and Techniques. Proc. 22nd European Conference on Optical Communication. ECOC'96. ThB. 2.1, 1996.
The nonlinear loop mirror and the wavelength converter can shape the waveform of the input optical signal. However, they cannot improve the S/N ratio sufficiently when there is large amount of noise (for example, ASE (Amplified Spontaneous Emission)). It is particularly difficult to improve the S/N ratio when there is large amount of noise on the marked level of the input signal. Here, the “marked level” is the emission state where the optical signal consists of the emission state and the extinction state (or the low emission state). The extinction state is, meanwhile, sometimes referred to as the “spaced level” or “base level”.
Non-patent Documents 1 and 2 disclose a configuration with multistage nonlinear loop mirrors or wavelength converters, and state that such multistage configurations yield better waveform-shaping effects compared with single-stage configurations. However, even with a multistage configuration, when the noise level is high and the S/N ratio of the input signal is significantly reduced, the waveform shaping has little effect, and consequently the signal waveform may be, nevertheless, distorted.